In order to be useful in fuel cells such as Solid Oxide Fuel Cells (SOFCs), anodes (fuel electrodes) must possess a high performance in terms of high electrochemical activity and high redox stability. Current state of the art Ni—YSZ anodes provide a reasonable electrochemical activity at high operating temperatures, often above 800° C., but are normally not redox stable. Volume changes in Ni—YSZ anodes due to reduction and oxidation of Ni results in inexpedient mechanical stresses in the anode material which impair the performance of the fuel cell.
In “Ni/YSZ and Ni—CeO2/YSZ anodes prepared by impregnation of a solid oxide fuel cell”, Journal of Power Sources, Qiao et al. disclose the preparation of Ni—CeO2/YSZ anodes by tape casting and vacuum impregnation. The addition of CeO2 is said to enhance cell performance.
U.S. Pat. No. 5,350,641 Mogensen et al. discloses the use of CeO2-based ceramics as the anode in a fuel cell.
U.S. Pat. No. 6,752,979 Talbot et al. discloses the preparation of nano-sized ceria particles with templating surfactants. The removal of the surfactant and attendant formation of nano-sized particles having grain sizes of 2-10 nm is effected by calcination at e.g. 300° C.
In “Mesoporous thin films of high-surface-area crystalline cerium dioxide”, Microporous and Mesoporous Materials 54 (2002), 97-103, Lunderg et al. disclose the formation of nano-sized ceria particles by the removal of templating surfactant during calcination at about 400° C.
According to conventional preparation methods, metal supported cells have been manufactured by co-sintering of a metal support tape in contact with a Ni-containing anode tape. This has resulted in extensive alloying/mixing of Ni, Cr, and Fe in the anode layer directly dependent on the sintering temperature. Co-firing a Ni-based anode at high temperature in reducing atmosphere also leads to coarsening of the Ni particles to unacceptably large particle size. This can result in poor performance of the catalyst and changes in the thermal expansion coefficient, mechanical properties or oxidation resistance of the metal support. Additionally, this type of anode layer partially oxidises under operating conditions and leads subsequently to expansion of the anode layer and eventually electrolyte rupture.
WO-A-2005/122300 describes metal supported anode structures manufactured from powder suspensions containing FeCr alloy, a layer for anode impregnation comprising ScYSZ and FeCr alloy, an electrolyte layer. The thus obtained half-cells are sintered and a solution of Ni, Ce, Gd nitrates is impregnated into the anode layer by vacuum infiltration thus resulting in an anode containing 40 vol % Ni. A cathode layer is subsequently deposited on the electrolyte surface.
WO-A-2006/116153 discloses a method of forming a continuous network of fine particles on the pore walls of a porous structure in a single step by removing the solvent of a solution containing a metal salt, surfactant and solvent prior to infiltration. The removal of the solvent is conducted by heating.